Runoff water from parking lots and filling stations is cleaned by first passing via an inlet pipe tangentially into a vortex chamber where the swirling action separates the contaminants from the water. Floating contaminants are stored in the vortex chamber, trapped at the top of the chamber by the roof of that chamber and at the lower part of the chamber by a skirt. That roof has an access opening to permit removal of the floating contaminants. The body of the fluid thereafter moves downward into a settling chamber where heavier-than-water contaminants settle. As the fluid in the settling chamber builds up, it passes through cleaning screens to a clean water outlet. A by-pass overflow pipe to process very large volumes of water extends upward from the settling chamber. Each screen may have louvers for preventing contaminants from blocking the screen.